Audio systems often include at least two speakers positioned to the front left and right of a listener. In a conventional home theater setting, the two speakers are positioned to the left and right of a television screen. In a conventional desktop setting having a computer, the two speakers are positioned to the left and right of a computer screen. Optionally, bass units or subwoofers are added, which can deliver the lower frequency range and permit smaller speakers. The subwoofer can be placed at almost any location. An optional center speaker may be positioned directly in front of the listener, between the left and right speakers. A conventional audio system having left and right speakers and a subwoofer (known as a “2.1” configuration) and the optional center speaker (known as a “3.1” configuration) is compact, but does not provide surround sound to the listener.
One conventional approach to providing surround sound to the listener is to add rear speakers to the listening area, located behind or to the sides of the listener. The speakers form a circular, square or rectangular array with the listener at or near the center. A surround sound processor may incorporate positional encoding by means of specific phase differences and amplitude ratios to enhance the effect of spatially surrounding the listener. A five speaker arrangement with a subwoofer (known as a “5.1” configuration) provides a desired surround sound effect. It is desirable, however, to achieve a surround sound effect with a more compact configuration.
U.S. Pat. No. 5,109,416 to Croft discloses a dipole speaker for producing ambient sound in a multichannel sound reproduction system. Used in conjunction with direct path speakers connected to the audio system, the additional surround dipole speakers are placed in front of and behind the listener on the centerline of the listening setup. The radiation from the dipole speaker is such that one lobe of the sound pressure output is 180 degrees out of phase with the other so that they cancel each other out to produce a null zone aligned towards the listener. The sound pressure lobes are directed toward the walls so the acoustical signal of the dipole is reflected therefrom and arrives at the listener by an indirect path. The delayed indirect path of the signal from the dipole enhances the ambience of the effect of the dipole speaker. However, Croft does not achieve a compact design because a rear dipole speaker is required directly behind a listener. If the listener is using the Croft system with a computer on a desktop, the use of a rear dipole speaker may not be feasible.
One conventional system attempts to provide a system with a surround sound effect for use with a computer. U.S. Pat. No. 5,809,150 to Eberbach discloses a surround speaker system that utilizes skewed hypercardioid sound energy fields from right front and left front “surround” speakers with the principle nulls directed at the expected listener location. This produces the effect of side wall and rear wall speakers in a home theater setting without actual side wall or rear wall speakers. The effect is enhanced by secondary nulls that are directed so as to reflect off the front wall of the room toward the expected listener location. Each surround speaker contains an antiphase driver and circuitry that powers the drivers to create the skewed hypercardioid sound energy field. However, Eberbach's hypercardioid sound energy fields do not produce a sufficient dispersion of sound. With a very tight pattern, hypercardioid speakers do not provide a wide distribution across the horizontal plane. As a result, the surround sound effect is minimized.
Conventional surround sound systems generally require rear speakers for proper production of a surround sound effect. It would be desirable, therefore, to have a system which is capable of producing such surround sound effect without speakers positioned in the rear of the listening area.